Circadian periodicity is a widespread, if not universal, phenomenon in the living organisms. In humans most of the available circadian information concerns renal and adrenal function. The information, however, is largely descriptive especially in regard to renal electrolyte cycles. Investigation of the mechanisms underlying the normal diurnal rhythms and its pathological deviations, a knowledge of their significance to the physiology and the pathology of fluid and electrolyte metabolism are sorely needed. The applicant's general plans are to devote his research efforts to such an investigation. He believes that because of his background in renal and adrenal physiology and because of the facilities presently available (automated chemical analysis, electronic computer, clinical research centers and most recently radioimmunoassay procedures in his own laboratory) the diurnal approach is feasible and likely to give fresh insights into the important outstanding problems of salt and water balance (e.g. salt excreting factor, hypertension, particularly the group with low plasma renin activity, etc.). For reasons explained in the application, the major effort must concern human volunteers and the patients; however, a search for experimental models of renal rhythms should be made and it is planned, since they are so useful in the study of mechanisms. As an approach to this investigation, a postural protocol under different conditions of salt balance is submitted. Correlation of hemodynamic and hormonal factors, simultaneously measured, will allow, it is hoped, a deeper insight in the diurnal rhythms. An additional powerful tool for this investigation is a newly established mathematical procedure (Pattern Recognition) for an objective classification of circadian data.